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; 4 Document Number: OCAS-TIB-O1L
Technical Information Bulletin

| Effective Date: (4/15/2005) — Revision No. 1 | Page 5 of 7
1
Bi-212 60.6 min Alpha decay (36%), Beta deay (64%)
Po-212 304 nsec Alpha decay
TI-208 3 min Beta decay

When the chain is in 100% equilibrium, the activity of each generation is equal.
However, the number of atoms required to reach this equilibrium activity varies
depending on the half-life. Since the working level is defined by the ultimate energy
released, it is affected by the number of atoms more than the total activity in a mixture.
The alpha energy is emitted only by Po-216 and essentially Bi-212. In reality Bi-212
emits an alpha only 36% of the time but the other 64% of the time it emits a beta then an
alpha almost immediately through the decay of Po-212 (304 nsec half-life). The average
alpha energy released by a Bi-212 decay is then 7.8 MeV (6.07 Mev * 0,36 + 8.785 MeV
* 0.64). Since Po-216 ultimately decays by two alphas (Po-216 and Bi-212) the alpha
energy released per Po-216 atom is 14.6 MeV. Pb-212 and Bi-212 both decay with an
effective alpha energy of 7.8 MeV. Po-212 decays with an energy of 8.785 MeV and Tl-
208 decays by beta decay only.

The largest number of atoms associated with the chain in equilibrium are Pb-212 and Bi-
212 due to the relatively long half-lives. ‘The remaining nuclides contribute little to the
WL measurement or the lung dose.

Since Pb-212 has a half-life approximately 10 times larger than Bi-212, the number of
Pb-212 atoms in an 100% equilibrium mixture is approximately 10 times that of Bi-212.
This causes Pb-212 to be the primary contributor to the WL measurement regardless of
the degree of equilibrium. On the other hand, Pb-212 atoms inhaled have more time to be
removed from the lungs prior to emitting all energy that will ultimately be emitted by the
remainder of the decay chain. This causes Pb-212 to produce less lung dose per unit
activity inhaled than that of Bi-212. Therefore, while the amount of Pb-212 in air drives
the WL measurement, the amount of Bi-212 in air is a large contributor to lung dose.
This causes the degree of equilibrium to be an important parameter. A good
measurement of this degree of disequilibrium is the ratio of Bi-212 to Pb-212.

‘The degree of equilibrium depends entirely on the circumstances surrounding the
production and movement of the Rn-220 gas in the breathing zone. This cannot be easily
predetermined but a range of likely values can.

ICRP 32° indicates this ratio (Bi-212 to Pb-212) is between 0.2 and 0.8. Pillai and Paul’
indicate the ratio is between 0,11 and 0.37 in a monazite mine and 0.24 to 0.84 in open
air. This calculation will use 0.8 as a default value but also provide values for a range of
equilibrium values, This will allow the most appropriate value to be used for the
particular circumstances.



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c) Particle Size

The ICRP recommends a default particle size of 5 micron for a working environment and
1 micron for members of the public exposed to environmental releases. In both cases,
these particle sizes are the median of a lognormal distribution of particle sizes. The
default geometric standard deviation (GSD) is 2.5, the default density is 3 g/cc and the
default shape factor is 1.5. This calculation uses all these defaults with the exception of
the particle size.

‘The particle size distribution affects the regional deposition of the aerosol in the lungs.
This size is greatly affected by the unattached fraction. The fraction that is attached to
other particulates can exhibit the aerodynamic characteristics of the larger particle to
which it is attached, The unattached fraction, on the other hand, represents very small
particles produced from the noble gas (radon).

Nikezic et, al.° indicated a particle size distribution for the attached fraction of 0.2 micron
with a GSD of 2.35 while the unattached fraction was 0.0015 micron with a GSD of 1.1.

Porstendorfer® indicated particle sizes for various environments. The unattached median
particle sizes ranged from 0.00013 micron to 0.0013 micron with GSDs ranging from 1-1
to 1.3. For the attached fraction, the major contribution (~86%) exhibited median particle
sizes of 0.33 micron for outdoor air and 0.217 micron for indoor air. Most of the
remaining distribution exhibited a median size of ~0.040 micron. The GSD for these
distributions varied from 1.5 to 2.2.

Birchall and James! indicated that the available data of attached aerosol size in
underground uranium mines indicates a particle size between 0.15 micron and 0.5 micron
with Samet et. al.” considering the best estimate to be 0.25 micron.

ICRP 32? indicates the value between 0.2 and 0.3 micron.

Based on the references above, this calculation will use a median particle size of 0.25
micron for the attached fraction. A size of 0.0015 micron will be used for the unattached
fraction. The ICRP 66 default value of 2.5 will be used for the geometric standard
deviation.

d) Equilibrium

The last major factor to consider is the state of equilibrium exhibited by the decay chain
inhaled. The decay chain of Rn-220 is shown below.

Rn-220 55.6 sec Alpha decay
Po-216 0.15 sec Alpha decay
Pb-212 10,64 hours Beta decay


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6.0 Results

The ICRP 66 lung model was solved using the assumptions described above. Under the
default assumptions used in this report, a dose of 7.61 rem to the lungs will be assigned
per WLM of Rn-220 decay products inhaled. Similarly, the default doses to the ET1
region will be 1750.33 rem and 2.90 rem to ET2. This value should be considered to be
received entirely in the year the material was inhaled.

The values for different degrees of equilibrium were also calculated and are provided in
the table below. The degree of equilibrium is specified by the ratio of Bi-212 activity to
Pb-212 activity. These values can be used is information for the specific situation is
available.

____ Table led for various degrees of Equilibrium
% equilibrium iB m) | ET1 dose (rem) ET2 dose (rem)
| 212/Pb-212) _ _| per WLM per WLM __| per WLM
| 20% ears ee 932.99 | 0.17
| 40% 5.27 1215.58 - 1.11
60% 6.46 1487.84 2 2.03
[ 80% ERO 1750.3: 2.90



Office of Compensation Analysis and Support Document Number: OCAS-TIB-011
Technical Information Bulletin Effective Date: (4/15/2005)
Revision No, |
Lung Dose Conversion Factor for Thoron WLM Page | of 7

W. Ihe Date: Yrof Supersedes: None

Approval:
J.W. Negn| Associate Director for Science |
ES |
Sis ~__ RECORD OF ISSUE/REVISIONS i |
Is! i EFFECTIVE REY.NO. DESCRIPTION
AUTHORIZATION DATE
DATE
New document to provide a basis fur the
3/16/2005 0 ___ conversion factor. - ‘
4/5/2005 Correction factors added for ET1 and ET2 regions |

1.0 PURPOSE

This Technical Information Bulletin provides the conversion factor for calculating lung
dose from Rn-220 decay products measured in working level months (WLM).

2.0 Background

EEOICPA uses the Interactive RadioEpidemiological Program (IREP) to determine the
Probability of Causation (POC) that a cancer was caused by occupationally related
radiation exposure. Annual radiation dose is the normal [REP input but in the case of
radon, a direct exposure model is also included for lung cancers. The exposure model
was based on studies performed at uranium mines and thus applies to Rn-222 and its
associated decay products. Rn-220 (also known as thoron) decay products have
characteristics that are sufficiently different from Rn-222 so that the exposure model is
not applicable to thoron. However, thoron decay products are often measured in units of
working levels. This requires the development of a conversion factor to express thoron
working level exposures in terms of lung dose in rem.

3.0 Working Level Definition

The majority of dose received from radon is actually derived from energy emitted by the
decay products rather than the gas itself. Because of this, the dose per unit radon gas
activity is greatly dependent on the state of equilibrium of the decay products. To better
describe the hazard associated with radon, worker exposure has historically been
expressed in terms of the decay product activity. The unit developed for this is the
Working Level. A Working Level is described as the amount of short lived radon decay
products in one liter of air that will ultimately decay with 1.3E5 MeV of alpha energy.

When the decay chain reaches 100% equilibrium, one Working Level is equivalent to
approximately 100 pCi/L of Rn-222 or 7.5 pCi/L of Rn-220. While this is a significant


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7.0 References
L. Birchall A., James A. C., Uncertainty Analysis of the Effective Dose per Unit

is)

6.

10.

Exposure from Radon Progeny and Implications for ICRP Risk-Weighting
Factors, Radiation Protection Dosimetry vol. 53, pp 133-140 (1994)

Hursh, J. B., Schraub, A., Sattler, E. L. and Hofmann, H. P. Fate of~ *!2Pb Inhaled
by Human Subjects. Wealth Physics 16, 257-267 (1969)

ICRP Publication 32, Limits for Inhalation of Radon Daughters by Wor.
ICRP Vol. 6 No, 1 (1981)

ICRP Publication 66, Human Respiratory Tract Model for Radiological
Protection. ICRP Vol. 24 No. 1-3 (1994)

Marsh J. W., Birchall A., Determination of Lung-To-Blood Absorption Rates For
Lead and Bismuth which are Appropriate for Radon Progeny. Radiation
Protection Dosimetry vol. 84, No. 4. pp 331-337 (1999)

Nikezic D. Haque A. K. M. M., Yu K.N. Effects of Different Deposition Models
on the Calculated dose Conversion Factors for”? Rn Progeny. Journal of
Environmental Radioactivity vol 61, issue 3, pp. 305-318 3 (2002).

Pillai, P. M. B., Paul, A. C. Studies of the Equilibrium of ?”’Rn (Thoron) and its
Daughters in the Atmoshphere of a Monazite Plant and its Environs. Radiation
Protection Dosimetry vol. 82, No. 3, pp 229-232 (1999)

Porstendorfer J., Physical Parameters and Dose Factors of the Radon and Thoron
Decay Products. Radiation Protection Dosimetry vol. 94, No. 4, pp 365-373
(2001)

. Samet, J. M., Albert, R. E., Brain, J. D., Guilmette, R. A., Hoopke, P. K., James,

A.C., and Jaufman, D. G. Comparative Dosimetry of Radon in Mines and Homes.
National Academy Press (1991).

Yamasaki, T., Guo, Q., Jida, T. Distributions of Thoron Progeny Concentrations
in Dwellings Radiation Protection Dosimetry vol. 59, No. 2, pp 135-140 (1995)



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ae A . - D ent Number: OCAS-TTB-011
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Effective Date: (4/15/2005) — Revision No. ! Page 2 of 7

difference in the gas concentrations, by definition of the working level, the de
products will ultimately emit the same amount of alpha energy. This, however, does not
lead to the same lung dose. This is caused largely by the difference in the half-lives of
the decay products. Since Rn-220 products have longer half lives, the body has longer to
transport the activity from one compartment to another and thus deliver dose to different
areas of the lung.

Since the working level is an air concentration, it is analogous to an exposure rate. To
describe exposure, the unit working level month (WLM) is normally used. A WLM is
described as the exposure equivalent to breathing one WL for 170 hours (one working
month),

4.0 Calculation Method

The ICRP 66% lung model was used to determine the proper conversion factor of dose per
WLM Rn-220, The model consists of 14 compartments, Material deposited in the lung
is moved among these compartments as well as removed to the blood stream, the GI tract,
and via extrinsic means. No absorption occurs in one compartment, (ETL).

Default absorption rates are described as a combination of two rates, a rapid rate and a
slow rate as well as the fraction of material that is absorbed rapidly. This requires the
model to use 2 compartments for each of the 13 lung compartment from which absorption
occurs, Therefore, the mathematical model consists of 13 rapid absorbing compartments,
13 slowly absorbing compartments, and one other compartment for a total of 27
compartments.

To further complicate the model, Rn-220 decays to 5 important decay products and the
model must be solved for each. This then requires a total of 135 mathematical
compartments in order to solve the model.

Each compartment can be solved analytically using simple first order differential
equations. The solution to these equations becomes tedious when the compartment
receives material that has passed through several compartments previously or has
decayed through a chain of several nuclides to reach that point. Therefore, several
simplifying assumptions have been made. Two of the decay products (Po-216 and Po-
212) have very short half-lives (<Isec). Po-216 was considered to be in equilibrium with
Rn-220 gas. Po-212 is produced in 64% of the Bi-212 disintegrations and was therefore
each Bi-212 decay was considered to also immediately produce a Po-212 decay.

5.0

del Parameters

The choice of parameters for the model is an important issue, There are numerous
parameters to define. The ICRP defines default parameters for the majority and these are
to be used unless more specific information is known about the particular situation.



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Parameters chosen specifically for Rn-220 decay product inhalation are discussed in
more detail below. The remaining parameters used the ICRP defaults.

a) Absorption

The ICRP 66° lung model defines three default categories for particulate lung absorption.
These defaults are to be used unless more specific information is known about the
situation at hand. In the case of radon decay products, a number of references are
available indicating an absorption half-life of approximately 10 hours. Marsh and
Birchall? describe an evaluation while Nikezic’ lists four separate references to this value.
Therefore, the 10 hour half-life is used in this calculation.

b) Unattached Fraction

Radon decay products are normally electrically charged. This causes the products to
quickly attach to particles normally in the air. This greatly affects the aerodynamic
characteristics of these particles which in turn affects the deposition of the particles in the
lungs. While unattached fraction is not specifically a parameter of the [CRP 66 lung
model, the aerodynamic parameters are.

Birchall and James' used an unattached fraction of 1% based on a report by Samet et. al?
Nikezic et. al.° used 8% apparently based on the same report. The largest difference
appear to be that Birchall and James attempted to describe typical mine air while Nikezic
attempted to describe typical household air. It should also be noted that both reports were
describing Rn-222 decay products. The unattached fraction of Rn-220 decay products
can be different due to the greatly different half-lives of the products. The Rn-220
products have longer half-lives and thus a longer period of time for the attachment to
occur.

Yamasaki et. al,'” performed a theoretical calculation that showed the unattached fraction
for Rn-220 decay products to be approximately 1.6% for indoor air.

Porstendorfer® demonstrated showed the unattached fraction depends on the
concentration of particulates in the air. He also showed that the unattached fraction of
Rn-220 decay products is smaller than that of Rn-222 decay products. His values match
well with each of the references above therefore, this calculation will use a value of 2%.
It is likely that the dust concentration in a work environment is higher than in a dwelling
and thus the unattached fraction should be lower for the work environment. Therefore,
when Yamasaki’s value of 1.6% is considered, the 2% unattached fraction should be
slightly high for a standard work environment, This can be shown to be a slight
overestimate of dose conversion factors but with the percentages so low, it appears to be
a reasonable,